1. Field of the Invention
The present invention relates to a method and apparatus for detecting a lane on a road, more particularly to the method and apparatus for detecting a lane on a road by defining boundaries of the opposite sides of the road, on the basis of image information in a visible region corresponding to a field of view of a vehicle driver.
2. Description of the Related Arts
In general, in order to define boundaries of a road geometrically, positions corresponding to white lines at the opposite sides (right and left sides) of the road are found. In practice, the positions having a common normal directed toward the center of curvature on a planar surface of a road are determined, so as to obtain tangents to the right and left white lines. An intersection of the normal with the tangent is called as a "corresponding point", and a line segment between the right and left corresponding points is called as a "road segment". If a plurality of road segments are obtained at a plurality of positions on the road, and arranged in parallel with each other, the road segments will form a plane corresponding to the surface of the road, and each segment directs a direction of the road at the position of the segment.
With respect to the above-described method for defining the boundaries of the road, an article entitled "A method to extract corresponding points in road image analysis" is disclosed in the papers of electronic information and communication committee, D-II Vol. J72-D-II No.5, pp.827-830, published in May, 1989. In that article, is proposed a method for obtaining a tangent vector T of a lane, then obtaining a unit segment vector S which is a normal to the tangent vectors at the right and left sides of the road, and seeking a combination of the tangent vectors to satisfy S.multidot.T=0, as shown in FIG. 19.
Another method is proposed in a paper entitled "A ZERO-BANK ALGORITHM FOR INVERSE PERSPECTIVE OF A ROAD FROM A SINGLE IMAGE" (Proc. IEEE Interl. Conf. on Robotics and Automation, pp.1444-1449, published in March, 1987). According to the method proposed in that paper, if M1-M2 and N1-N2 in FIG. 20 are found to be parallel, the other end of the initial cross-segment is taken to be the intersection R of the normal with the other edge segment M1-M2. If the two edge segments are not parallel, as can happen in turn as shown in FIG. 20, a kind of center of curvature C of the turn is found as the intersection of the normals to the midpoints of the two segments, and the width w of the road is calculated as the difference between CQ and CP. Then, the endpoint R of the initial cross-segment is taken at a distance w from its endpoint Q.
In the mean time, an image processing system is proposed for processing image information in a visible region in front of an automotive vehicle so as to be applied for various uses. According to Japanese Patent Laid-open Publication No. 6-215137, for example, it is proposed to provide an image processing system for detecting edge points in the image rapidly, which system is adapted to determine a scanning region by combining scanning line information such as a coordinate for beginning the scanning process and a coordinate for ending the scanning process, which are stored in a scanning table memory device. In practice, in order to detect boundaries of a white line extending ahead of a vehicle, theoretically it is sufficient to scan image data in a relatively narrow region to obtain edge points of the white line, because the region where the white line resides is generally limited to a certain narrow region. According to the proposed system, therefore, a scanning region which is called as a window is determined in accordance with the combination of the scanning line information such as the coordinate for beginning the scanning process and the coordinate for ending the scanning process, which are stored in a scanning table memory device.
With respect to the method for defining the boundaries of the road, the method disclosed in the former article requires repetition of the calculations until S.multidot.T=0, so that the response is to be improved. In the method disclosed in the latter paper, a small block of a curve road is represented by a trapezoid region, and the trapezoid region corresponding to the nearest road segment is added by next trapezoid region to extend the road segments. Therefore, if the road is dirty or the white lines are partially disappeared, the road segment may not be formed, so that the road segments can not be extended.
In the prior publications as described above, the boundaries of the road or the road segments are used. In general, a couple of parallel white (or other clear colours) lines are painted on the road to define the right and left boundaries of the road, so that a lane along which a vehicle travels is provided between the white lines. In other words, the lane of the road can be detected by defining the two white lines. In the present invention and embodiments, therefore, "lane segments", or simply "segments" are used in lieu of the road segments, and "boundaries of the lane" is used in lieu of the boundaries of the road. If the white lines are painted on the edges of the road, the lane is the same as the road.
In order to detect the lane shape of the road stably and accurately, it is desirable to set a plurality of lane segments from the nearest position toward a remote place therefrom with a constant distance spaced from each other, as described above. However, the white lines are not necessarily painted to be solid lines, but sometimes painted to be broken lines. Therefore, in the case where the road segment is set only in a predetermined limited region with respect to the image in front of the vehicle which travels on the road, if the white lines do not reside in that region, the road segment can not be set.